Arrested growth of the largest UK European Storm-petrel colony Resurvey reveals arrested population growth of the largest UK colony of European Storm-petrels Hydrobates pelagicus, Mousa, Shetland Mark Bolton*, Danaë Sheehan, Susannah E. Bolton, Jane A. C. Bolton and Jack R. F. Bolton * Correspondence author. Email: [email protected] 1 RSPB Centre for Conservation Science, UK Headquarters, The Lodge, Sandy, Bedfordshire SG19 2DL, UK. Abstract Playback resurvey of the UK’s largest breeding colony of European Storm-petrels Hydrobates pelagicus at Mousa, Shetland revealed that the substantial population growth achieved during previous decades has not been maintained. The estimated population size in 2015 was 10,778 apparently occupied sites (AOS) (95% confidence limits [CL] 8,857–13,207). The mean nesting density of birds breeding in natural habitat had declined substantially compared with 2008, but the area occupied by most sub-colonies had increased. Comparison with sub- colonies surveyed in 2008 indicated a 12.8% decline, though the lack of precision that surrounds both surveys renders the decline statistically non- significant. We discuss the possible causes of the observed change in trend. The use of a new playback recording that did not include alarm calls was associated with a substantial increase in response rate compared with previous surveys. Daily response rates from nests of known occupancy status declined during the course of the fieldwork period, associated with increasing absence of adults at the nest during daylight, as chicks acquired thermoregulatory independence and adults remained at sea by day. We therefore use a date-specific calibration factor to estimate AOS density. Methods of data analysis were improved for the current survey to allow estimation of the number of AOS and associated CL for each sub- colony separately. This resulted in a 51% reduction in the size of the confidence interval of the colony population estimate relative to the mean, compared with the 2008 survey. Playback surveys of burrow-nesting seabirds are typically characterised by low precision, which hinders statistically robust detection of population change, even when large declines are indicated. We recommend the adoption of a playback recording that does not include alarm calls, which may depress the frequency of responses or their detection by observers. Further, we suggest that for colonies where sub-colonies occur in discrete patches of habitat that are likely to vary in nesting density, the number of AOS should be estimated for each sub-colony separately. Adoption of these small modifications could substantially improve precision of playback surveys and hence the power to detect population change. SEABIRD 30 (2017): 15–30 15 Arrested growth of the largest UK European Storm-petrel colony Introduction European Storm-petrels Hydrobates pelagicus breed on islands of the northeast Atlantic and Mediterranean with the largest populations occurring in the Faroes, Iceland, Ireland and the UK. The global population is estimated at around 1.5 million individuals (Brooke 2004). Although the current status of many colonies is not well known, the species is suspected to be in decline globally owing to predation by invasive species, pollution and development at breeding localities (Birdlife 2016). The first comprehensive, quantitative assessment of the breeding status of European Storm-petrels in Britain and Ireland was conducted during the Seabird 2000 census (Mitchell et al. 2004), which concluded that Britain and Ireland jointly held around 83,000 pairs in 95 surveyed colonies, with other unsurveyed colonies probably supporting several tens of thousands of additional pairs. Together the populations of Britain and Ireland represent 14–54% of the biogeographic population of the subspecies H. p. pelagicus. Since the Seabird 2000 census, rat eradication programmes have led to the establishment of several further colonies in the UK on islands that were formerly unsuitable (Ramsey, Lundy, St Agnes and Gugh). Notwithstanding these successes, resurvey of some of the largest UK colonies in recent years have indicated declines, (e.g. Priest island, Ross and Cromarty, West Scotland; Insley et al. 2014). The largest UK colony at the time of the Seabird 2000 census was Mousa, Shetland, holding 5,410 pairs in 1996 (estimate recalculated by Bolton et al. 2010), representing about 22% of the UK population at that time. Resurvey in 2008 indicated a substantial increase to 11,781 pairs (Bolton et al. 2010). Here we report results of a census conducted in 2015 and consider possible causes of the changes observed since 1996. Methods Study site: Mousa (60°00’N, 01°11’W) is an uninhabited 180 ha sheep-grazed sandstone and limestone island lying 1 km off the southeast Shetland mainland. The island is designated as a European Special Protection Area (SPA) within the Natura 2000 network, reflecting the international importance of the site for breeding European Storm-petrels and Arctic Terns Sterna paradisaea. European Storm-petrels nest in crevices in storm beaches and under loose scattered boulders and piles of stone slabs from abandoned quarry workings, as well as in dry stone walls, ruined buildings and a 2000-year old Iron-age broch. Storm-petrels are highly vulnerable to predation and are active at the colony only during darkness, when males sing from their nest cavities. The only mammalian predator present on the island is the European Otter Lutra lutra (hereafter “otter”), with several active holts and evidence of predation on Storm-petrels in several parts of the island (M. Bolton, pers. obs.). Other predators of Storm-petrels present on the island are Great Skua Catharacta skua (30–40 pairs), Arctic Skua Stercorarius parasiticus (1–8 pairs), Herring Gull Larus argentatus (c. 5 pairs), Lesser Black-backed Gull L. fuscus (c. 1 pair) and Great Black-backed Gull L. marinus (c. 16 pairs). 16 SEABIRD 30 (2017): 15–30 Arrested growth of the largest UK European Storm-petrel colony Playback protocol: The standard survey protocol for Storm-petrels relies on diurnal playback of song at potential nesting locations to elicit responses from nesting birds (Ratcliffe et al. 1998). A recording of male purr call was played at approximately 75 dB for 10 seconds, and the number of apparently occupied sites (AOS) from which a vocal response of an adult was elicited within 30 seconds, was counted. Typically, adults respond to playback with a single brief alarm call, rather than purring song. The analogue recording of Storm-petrel song from Mousa that was used for both earlier surveys contained several alarm calls during the 10 seconds, which may have contributed to the low response rate recorded in earlier surveys, either by inhibiting responses from nest-holders, or by masking genuine alarm responses. For the present survey, a new digital recording was prepared, using a 10 second recording from Mousa of male purr song that did not contain any alarm calls. Any alarms heard by the observer would therefore be correctly attributed to a response, rather than to the playback recording. Playback recordings were presented using a LOGIC MP3 player (model L2GMP309) and RadioShack® 200 mW mini amplifier speaker (Model 2771008). Response rate is known to vary diurnally, with highest response frequencies close to sunrise and sunset. In common with previous surveys, fieldwork was conducted between 06:00 hrs and 18:00 hrs GMT, when response rate varies little (Ratcliffe et al. 1998). Location and extent of sub-colonies and suitable habitat: The earlier resurvey in 2008 was conducted on the assumption that the location and extent of areas occupied by sub-colonies had not changed since the initial survey in 1996. For the present survey we conducted nocturnal fieldwork to reassess the location and extent of sub-colonies in natural habitat and quarry workings. Sub-colony boundaries were determined from the location of singing birds and marked by landscape features or by handheld GPS waypoints. To enable direct comparison with earlier work, we followed precisely the survey methods employed by Ratcliffe et al. (1997). Parallel survey transects, each 1 m wide and spaced at 5 m intervals, were orientated perpen- dicular to the long axis of each sub-colony, extending across the entire area occupied by breeding birds. The area enclosed by the sub-colony boundaries was estimated as the length of the long axis multiplied by the mean transect length. Some areas within sub-colonies comprised solid rock, or grass without cavities or fissures and were therefore unsuitable for nesting. To estimate the extent of suitable breeding habitat within each sub-colony we multiplied the total area by the proportion of survey points that contained suitable habitat. Measurement of the total length of dry stone wall on the island from aerial photos indicated that it had remained unchanged since the 1996 and 2008 surveys at 4,923 m. Sampling strategy: Survey fieldwork was carried out between 22 July and 31 July 2015, when most birds were in the latter stages of incubation or brooding recently hatched chicks. To estimate the response density of birds nesting in dry stone walls, the same ten sample sections that had been selected for survey in previous years, each 100 m in length, were resurveyed. Playback was conducted at 1 m intervals along one side of each section and all AOS within 0.5 m of the survey point which elicited a response were counted. SEABIRD 30 (2017): 15–30 17 Arrested growth of the largest UK European Storm-petrel colony For sub-colonies in natural habitat and quarry workings, playback was conducted every 1 m along each transect. Responses from adults within 0.5 m of the transect line were noted, representing a sample of 20% of the entire area. Sample points which fell in locations that did not contain any habitat suitable for nesting were not surveyed. In previous surveys all occupied areas of natural habitat have been surveyed in this manner except for the large area of boulder beach to the north of the broch, which has been surveyed using four 5 x 5 m quadrats.